In recent years, an excellent multi-layer coating film with high saturation, high brightness and excellent color depth is known as an automobile body coating film, in which the excellent multi-layer coating film is formed by applying a color clear film containing a coloring pigment on a metallic base coating film containing a luster pigment. Such excellent multi-layer coating films are referred to as “candy color” coating film.
Candy color coating films are getting a lot of attention as an automobile exterior. However, making candy color coating films requires strictly control for coating conditions such as suppression of variations in film thickness so as to obtain uniform film design in whole automobile body having complex shape. Lack of control for coating conditions provides frequent color unevenness and non-uniform film design. In addition, suppression of variations in film thickness in a formation of coating film lowers production efficiency.
JP 2001-314807 A (Patent Document 1) describes a method for forming a multilayer coating film comprising a process to form a first coating film by applying a first coating composition containing a luster pigment to the surface of an object to be coated, a process to form a second coating film by applying a second coating composition which contains a coloring component whose content is 0.01-1 wt. % for the resin solid content of the coating composition to the surface of the first coating film, without baking and curing the first coating film, and a process to form a clear coating film by applying a clear coating composition to the surface of the second coating film, without baking and curing the second coating film, then the coating films are heated and cured. The method includes control of a content of the coloring pigment in the second coating film to resolve a problem in use of a color clear coating composition. The method can prevent color unevenness and color dulling of a coating film. Even if using such technologies, making a candy color coating film accompanies frequent color unevenness, difficulty of homogeneous film design and uniform coating color. In addition, suppression of variations in film thickness in a formation of coating film is required, which lowers production efficiency.
JP 2007-167720 A (Patent Document 2) describes a process for forming a multi-layer coating film comprising the steps of:
applying a metallic base coating composition on a coating substrate to form a metallic base coating film, wherein the metallic base coating composition comprises a luster pigment,
applying a color base coating composition to the metallic base coating film to form a color base coating film, wherein the color base coating composition comprises a coloring pigment,
applying a clear coating composition to the color base coating film to form a clear coating film, and
simultaneously heating and curing the metallic base coating film, the color base coating film and the clear coating film to form a multi-layer coating film, wherein a brightness L* value of the metallic base coating film is not greater than 60 and a light transmittance in a wavelength of 400 nm to 700 nm of the color base coating film is 30 to 50%. The Patent Document 2 also describes reduction of color unevenness due to variations in film thickness of a candy color coating film. However, the process is not enough to improve production efficiency. In addition, a candy color coating film having excellent color depth is required.